Optical data storage media including compact discs (CDs) and digital video discs (DVDs) contain physical relief structures that are used by an optical read/write head to obtain position and tracking information, error correction, conveying data content, etc. These relief structures are created during the optical disc manufacturing process and typically include very fine surface modulations. The surface modulations are often in the form of pits, bumps, grooves, or lands, etc., and can include features with submicron-scale down to nanometer-scale dimensions. The process of incorporating such features into the substrate of the optical medium during manufacturing is variously referred to as “preformatting” or “replication.” The benefits of preformatting include very efficient utilization of the optical disc surface, which in turn allows higher storage capacity and enhanced performance compared to media without such formatting.
The standard process for incorporating such information structures into the surface of optical disc substrates is injection molding, wherein pellets of a polymeric material, usually polycarbonate, are melted and injected into a mold containing an insert, or stamper, which has the mirror image of the desired pattern on its surface and against which the melted polymeric substrate material is brought into contact. Cooling of the mold allows the polymer material to solidify, at which point the substrate disc, now having the information formed into its surface, is removed.
In the case of optical media designed for recording/rewriting of user data, the molded features typically contain various format and header addressing information to help locate the user data, and additionally may contain other features, including those relating to prerecorded information, often referred to as read-only memory (ROM) information.
Although the injection molding process described above is the virtually exclusive manufacturing process used worldwide for making optical discs, a number of attempts have been made to fabricate data storage substrates by means other than injection molding, including continuous roll processing. For an article on continuous roll processing, see W. D. Slafer at al., “Continuous Web Manufacturing of thin-coverslip optical media”, SPIE Optical Data Storage '92, San Jose, Calif., 12 Feb. 1992. Such roll processes have involved a rotary or step-and-repeat replication process, either of which utilizes a replication tool whose surface contains the mirror image of the desired pattern, and which is used to create multiple replicas of the tool pattern.
There are a number of techniques known in the art whereby embossing tools used in the roll processes can be created, including direct etching on the external surface of the tool, or use of a flat substrate that is attached to the outer surface of a support member, patterned or patterned after mounting. See, for example, U.S. Pat. Nos. 5,521,030 and 5,147,763. It is further known that such tooling can be formed from discrete elements, whereby several substrates containing a pattern to be replicated can be joined, as in the form of a mosaic pattern, for example, and thereby increase the size of the tool.
Many of the techniques of the prior art described above include the inevitable creation of one or more joints or seams where substrate edges abut. The resulting master replication tool exhibits a discontinuity (seam) at the location at which the discrete pieces are joined. Such seams do not represent a limitation in applications in which the desired pattern to be replicated is by design composed of discreet elements (such as discs and cards), since the seam(s) can merely be placed between the natural divisions of these discrete elements. For other applications, the visual effects of the seam(s) can be reduced by techniques known in the art so as not to be noticeable or objectionable for a particular application, for example holographic designs or embossed foils for wrapping and packaging and the like. The discontinuities introduced by the existence of one or more seams, however, can present limitations in other applications in which a recording medium requires a format with a relatively long dimension compared to the replication tool, e.g., such as optical storage tape.
In addition to cosmetic effects, a discontinuity such as a seam that is made by the replication tool can often cause mechanical problems during the replication process, and techniques known in the art have been developed to reduce the seam, including careful piecing of the segments, grinding, polishing, etc. For some applications, such techniques are not acceptable, for example micropatterns that require a pattern without interruption for long distances, such as lenticules for photographic films and preformatted recording tape. See E. H. Land, “An Introduction to Polavision”, Photogr. Sci. Eng. 21: 225 (1977).
Examples of previous techniques of producing true seamless patterning tools have involved producing a mirror image of the desired pattern on the external face of a geometric surface, such as a drum, by engraving or by lithographic techniques, e.g., as taught in U.S. Pat. No. 6,045,980. These previous techniques, however, suffer from the drawback that the replica surface is often made of materials that are not durable (glass, photopolymer, etc.) or cannot be readily replicated. A direct (molded) copy of such an external master pattern would by necessity have the pattern facing inward, and attempting to “invert” such a tool, for example by slitting to turn it “inside out,” would by necessity introduce a seam.
Other examples of processes for making replication tools suitable for optical recording media involve spiral wrapping of a discreet tape over the outer surface of a sleeve to create an external pattern. See, for example, U.S. Pat. No. 5,872,758. This technique does not eliminate discontinuities, however, and suffers from the limitation of creating multiple diagonal seams on the substrate being imprinted.
What are desirable, therefore, are methods and apparatus that address the shortcomings noted for the prior art.